Let’s start our main discussions with exhaust sizes (NOT overall design, as we must first figure out what is required to meet engine requirements), dissect them a touch, and then decide what is needed and what will work.
You must also understand that when we talk “marine exhaust” we are usually talking a “wet” system, but also realize that inside this “wet system” we have two distinct parts. In most exhaust systems that are in the type of boats using 150-800 HP diesel engines, there are TWO distinct parts of the exhaust system/piping. The DRY part and the WET part. Even on the factory supplied 90 degree “wet elbow,” these two sections exist, though many people don’t realize it.
The inner pipe of this “wet elbow” is actually/usually a 90-degree dry bend, or section, surrounded by raw water to keep the surface cool. At the end of this “wet” elbow, where the exhaust hose attaches, is where the water is introduced (hence the term “mixing elbow”) and the exhaust NOW becomes wet. Inside this elbow is a smaller diameter (typically around 2 ½ – 6″ ID) which is the dry side, and where the hose attaches (it expands to (or is surrounded by) 4-10″ tubing/OD piping, depending on the engine size, etc.)
EXHAUST SIZE can be determined by a few rules, but all really come down to two things:
#1 – Meeting the engine requirements as to total restriction (back pressure).
#2 – Designing a system that will FIT inside the boat’s constraints.
Most Important Point to Understand – Very simple, if you can’t meet Rule #1 and Rule #2, then nothing else matters and the exhaust is the WRONG SIZE. One, it won’t meet engine specifications / requirements, and two, it won’t fit within the constraints of the boat.
But, back to sizing. Exhaust flow is determined from the amount of HP the engine makes and the more HP, the more exhaust flow; therefore it will take larger piping and/or less bends, shorter lengths etc., to meet back pressure requirements. Also, a very simple to understand concept (even though most installers seem to forget) is the “bend” equation part of planning an exhaust. Figure a smooth radius 90 degree bend is equal to about 6-10 FT of DRY piping and 15 FT of wet piping. Another good rule of thumb to remember is for every 100 PRODUCED horsepower, the engine makes about 200 CFM of exhaust gases, but this does not include the water and / or steam that becomes part of the mix when water is introduced. That’s why dry piping can be smaller than wet piping, WATER and steam add to the total flow in a substantial manner; therefore once water is introduced, the piping MUST BE LARGER.
Past experience as to what size will work and meet requirements is always a plus when in the initial planning. But not all are so lucky, so they turn to the engine specs or installation guide and see what is “recommended,” and/or they use the factory supplied wet elbow size to go by. In some cases this is fine and gets you by with the least amount of cost and effort. In most cases, the hose size that fits the factory elbows will work in 90% or more of the applications out there.
Contact DeAngelo Marine if you require an inspection / survey of your engine room, main engines or generator exhaust systems. We invite your inquiry for exhaust parts & accessories and exhaust repairs and service with your existing vessel. If you are in the design phase of new vessel construction or contemplating an engine overhaul, contact DeAngelo Marine Exhaust by email or by telephone: +1 954.763.3005.